The present invention relates to a switching power supply. In particular, the present invention relates to a switching power supply having a transformer which includes an input winding and an output winding, wherein a DC input voltage supplied to the input winding of the transformer is switched by a switching circuit.
Heretofore, various types of switching power supplies have been proposed and put to practical use. In a dominant type among them, an inputted DC voltage is switched by a switching operation of a switching circuit connected to an input winding of a power conversion transformer and the resulting switching output is taken out to an output winding of the power conversion transformer. A voltage appearing at the output winding based on the switching operation of the switching circuit is rectified by a rectifier circuit and then converted into DC by a smoothing circuit to be output.
In the switching power supply, an output rectifier diode is connected in series with a power transfer line. Thus, reducing the loss arising from this output rectifier diode may effectively contribute to an enhanced efficiency of the switching power supply.
The reduced loss of the output rectifier diode may be simply obtained by use of a diode having a low forward voltage drop. However, the diode having a low forward voltage drop involves in an insufficiently low reverse withstand voltage. Thus, particularly when the diode having a low forward voltage drop is used as the output rectifier diode, it is necessary to restrain the reverse voltage.
In this kind of switching power supplies, a most important consideration on the reverse voltage is a surge voltage arising from parasitic elements based on the switching operation of the switching circuit. The surge voltage is applied as a reverse voltage to the output rectifier diode. A snubber circuit has been known as means for restraining the surge voltage applied to the output rectifier diode. One conventional snubber circuit is described in FIG. 3 of Japanese Patent Laid-Open Publication No. Hei 6-54531 as a prior art. This known snubber circuit comprises a series circuit of a capacitor and a resistance wherein the series circuit is connected in parallel with an output rectifier diode. In order to restrain the surge voltage, this snubber circuit is adapted to consume unnecessary energy including the surge voltage by the resistance through the capacitor.
However, this conventional snubber circuit may not sufficiently restrain the surge voltage applied to the output rectifier diode when it is turned off. Thus, it is undesirably forced to use an output rectifier diode having a high withstand voltage. In addition, since the conventional snubber circuit is configured to consume the energy of the surge voltage by the resistance, an associated resistor is increased in size, resulting in undesirable heat and power loss in the resistor. Thus, this conventional snubber circuit involves a drawback in achieving a downsized and highly efficient switching power supply.
The aforementioned Japanese Patent Laid-Open Publication No. Hei 6-54531 also shows a snubber circuit using no resistor in FIG. 1 thereof. This snubber circuit disclosed in this Patent Laid-Open Publication comprises a series circuit composed of a capacitor and a diode and connected to both ends of an output rectifier diode, and an inductor connected between a connection point, at which the capacitor is connected to the diode, and an output side of an output smoothing choke coil. According to the snubber circuit having the above construction, unnecessary surge energy may be absorbed by the capacitor, and the absorbed energy may be regenerated at the output side of the switching power supply through the inductor.
However, in the aforementioned conventional snubber circuit, one end of the inductor is connected to the output side of the choke coil. Accordingly, the series circuit composed of the diode in the snubber circuit and the inductor is connected in parallel with the output smoothing choke coil. Thus, when the switching circuit is turned on, the current rectified by the rectifier circuit flows into not only the choke coil but also the diode of the snubber circuit and the inductor which are connected in parallel with the choke coil. This causes the loss due to a forward voltage drop in the diode of the switching circuit and the loss due to the DC resistance of the inductor. Further, this Patent Laid-Open Publication discloses the technique applicable only for single-diode type forward converter, and does not disclose any application to the switching power supply having a center tap type rectifier circuit. Assuming that the technique described in this prior art is employed in the switching power supply having a center tap type rectifier circuit as-is with departing from the disclosure therein, the demand for downsizing may not be satisfied due to the resultingly increased number of components.
It is an object of the present invention to provide a switching power supply having an enhanced efficiency yielded by reducing the loss due to a diode and inductor which form a snubber circuit.
It is another object of the present invention to provide a switching power supply capable of employing a small size, low power element to form a snubber circuit.
It is still another object of the present invention to provide a highly efficient, low noise, small size switching power supply, in a switching power supply having a center tap type rectifier circuit.
It is yet another object of the present invention to provide a highly efficient switching power supply capable of employing an output rectifier diode having a low forward voltage drop, in a switching power supply having a center tap type rectifier circuit.
It is yet still another object of the present invention to provide a switching power supply including a highly efficient, low noise snubber circuit capable of sufficiently restraining a surge voltage applied to a output rectifier diode, in a switching power supply having a center tap type rectifier circuit.
It is another further object of the present invention to provide a switching power supply including a low-energy-consumption, highly efficient snubber circuit, in a switching power supply having a center tap type rectifier circuit.
It is still a further object of the present invention to provide a highly efficient switching power supply having a snubber circuit capable of regenerating energy, in a switching power supply having a center tap type rectifier circuit.
It is additional object of the present invention to provide a small size switching power supply having a small number of components, in a switching power supply having a center tap type rectifier circuit.
In order to achieve the above and other objects, according to the present invention, there is provided a switching power supply comprising a transformer, a switching circuit, an output rectifier circuit, an output smoothing circuit, and a snubber circuit. The transformer includes an input winding and an output winding. The switching circuit switches a DC input voltage supplied through the input winding of the transformer. The output rectifier circuit includes at least one output rectifier diode. One electrode of the output rectifier diode is connected to one end of the output winding of the transformer. The input side of the output smoothing circuit is connected to the other electrode of the output rectifier diode.
The snubber circuit includes a snubber capacitor, a snubber diode, and a snubber inductor. The snubber capacitor and the snubber diode are connected with each other at each one end thereof. The other end of the snubber capacitor is led to the output winding, while the other end of the snubber diode is led to the other of the electrode of the output rectifier diode. One end of the snubber inductor is connected to a connection point between the snubber capacitor and the snubber diode, while the other end of the snubber inductor is connected to the input side of the output smoothing circuit.
In the aforementioned switching power supply according to the present invention, the switching circuit switches the DC input voltage supplied to the input winding of the transformer. The switching output is taken out on the side of the output winding of the transformer. The output rectifier circuit rectifies a voltage generated at the output winding of the transformer to provide an output. The output smoothing circuit smoothes the rectified output from the output rectifier circuit to provide an output. As a result, the switching output transferred to the output winding based on the switching operation of the switching circuit is rectified by the output rectifier circuit and then smoothed by the output smoothing circuit.
In a cycle in which the voltage appearing at the output winding of the transformer based on the switching operation of the switching circuit is the reverse direction to the output rectifier diode, the diode of the snubber circuit is arranged to have a polarity to form a charging loop circulating through the diode and capacitor provided in the snubber circuit, and the output winding of the transformer.
In this charging loop, an LC resonance circuit composed of the capacitor of the snubber circuit and a series inductance including a leakage inductance of the transformer and wirings is established, and thereby the terminal voltage in the capacitor of the snubber circuit is increased in accordance with a waveform of a LC resonance voltage. Thus, the surge voltage applied to the output rectifier diode is restrained in the voltage determined by the LC resonance voltage.
As described above, the present invention may restrain the surge voltage applied to the output rectifier diode so that a diode having a low forward voltage drop may be used as the output rectifier diode. Thus, the loss due to the output rectifier diode is reduced and thereby a low-energy-consumption, a highly efficient switching power supply may be obtained. In addition, since the surge voltage applied to the output rectifier diode is restrained by the LC resonance effect, a low noise snubber circuit may be achieved.
In the cycle in which the voltage appearing at the output winding of the transformer is the forward direction to the output rectifier diode based on the switching operation of the switching circuit, a discharge path to the snubber capacitor is established in the snubber circuit. This discharge path serves as a circuit for discharging the energy accumulated in the snubber capacitor through the snubber inductor. Since the snubber inductor is connected between the line, which connects the snubber capacitor with the snubber diode, and the input side of the output smoothing circuit, the energy accumulated in the capacitor is regenerated to the input side of the output smoothing circuit through the snubber inductor, which may provide an increased efficiency.
One important feature of the present invention is that the snubber inductor is connected between the line, which connects the snubber capacitor with the snubber diode, and the input side of the output smoothing circuit. According to this structure, the snubber inductor is never arranged in parallel with a choke coil of the output smoothing circuit. Further, the other end of the snubber diode in the snubber circuit is led to one electrode of the output rectifier diode connected to the input side of the output smoothing circuit. This brings both ends of the series circuit of the snubber diode and the snubber inductor into an electrically connected state, or a short-circuited state, at the input side of the output smoothing circuit. Thus, in the cycle in which the voltage appearing at the output winding of the transformer is the forward direction to the output rectifier diode based on the switching operation of the switching circuit, the current passing through the diode and inductor of the snubber circuit becomes extremely small. Thus, the loss due to the forward voltage drop in the diode and the loss due to the DC resistance of the snubber inductor in the snubber circuit are decreased to the extent that they may be substantially negligible.
As described above, since the current passing through the diode and inductor of the snubber circuit become extremely small, a low capacity, low power element may be used as the diode and inductor of the snubber circuit. As a result, the diode and inductor of the snubber circuit may be downsized, and thereby a smaller size switching power supply may be provided.
In another embodiment of the present invention, a switching power supply comprises a transformer including an input winding, an output winding and a center tap provided at the output winding, a switching circuit connected to the input winding of the transformer and switching a DC input voltage supplied to the input winding, a first output rectifier diode having one end connected to one end of the transformer, and a second output rectifier diode having one end which has the same polarity as that of the one end of the first output rectifier diode and is connected to the other end of the transformer, an output rectifier circuit in which the other end of the first output rectifier diode and the other end of the second output rectifier diode, which have the same polarity respectively, are connected with each other, an output smoothing circuit having an input side, which is connected to a connection point between the first and second output rectifier diodes and the center tap of the transformer, and an output side led to a pair of output terminals, and a snubber circuit. The snubber circuit includes a first snubber circuit, a second snubber circuit, and a snubber inductor. The first snubber circuit includes a first snubber capacitor and a first snubber diode, wherein the first snubber capacitor and the first snubber diode are connected in series with each other to form a series circuit. The series circuit is connected in parallel with the second output rectifier diode so as to allow the first snubber diode to have a reverse polarity to the second output rectifier diode. The second snubber circuit includes a second snubber capacitor and a second snubber diode, wherein the second snubber capacitor and the second snubber diode are connected in series with each other to form a series circuit. The series circuit is connected in parallel with the first output rectifier diode so as to allow the second snubber diode to have a reverse polarity to the first output rectifier diode. The snubber inductor having one end connected to the output side of the smoothing circuit and the other end led to the first and second snubber circuits to form a one-way discharging path for the first and second capacitors.
The first and second snubber circuits are different only in each operation timing but have substantially the same structure and operation respectively. In the cycle in which the voltage appearing at the output winding of the transformer is the forward direction to the first output rectifier diode based on the switching operation of the switching circuit, a charging loop circulating through the first output rectifier diode, the first diode, the first capacitor, and the output winding of the transformer is established. In the charging loop, an LC resonance circuit composed of a first capacitor and a series inductance including a leakage inductance and wiring of the transformer is established, wherein a terminal voltage of the first capacitor is increased in accordance with the waveform of the LC resonance voltage.
In the cycle in which the first output rectifier diode is conducted, a reverse voltage is applied to the second output rectifier diode. Since the series circuit composed of the first capacitor and the first diode is connected in parallel with the second output rectifier diode, a terminal voltage of the second output rectifier diode becomes substantially equal to the terminal voltage of the first capacitor when the forward voltage drop of the first diode is neglected. As described above, the terminal voltage of the first capacitor is increased in accordance with the waveform of the LC resonance voltage yielded by the first capacitor and the series inductance including the leakage inductance and wiring inductance of the transformer. Thus, the surge voltage applied to the second output rectifier diode is restrained in a voltage determined by the LC resonance voltage. Since the surge voltage applied to the second output rectifier diode may be restrained as described above, a diode having a low forward voltage drop may be used as the second output rectifier diode. Thus, the loss due to the second output rectifier diode is reduced, and thereby a low-energy-consumption, highly efficient switching power supply having a center tap type rectifier circuit may be obtained. Further, since the surge voltage applied to the first and second output rectifier diodes is restrained by the LC resonance effect, a low noise snubber circuit may be achieved. In the cycle in which the voltage appearing at the output winding of the transformer is the forward direction to the second output rectifier diode, i.e., in the cycle in which the reverse voltage is applied to the first output rectifier diode, based on the switching operation of the switching circuit, the second snubber circuit is actuated to restrain the surge voltage applied to the first output rectifier diode. Thus, the same effect may be obtained for the first output rectifier diode.
Further, the snubber circuit includes an inductor having one end connected to the output side of the output smoothing circuit and the other end led to the first snubber circuit to form a one-way discharging path for the first capacitor. According to this structure, in the cycle in which the switching circuit is turned off, the energy accumulated in the first capacitor may be regenerated to provide an improved efficiency. In addition, the inductor forms the one-way discharging path shared by the first and second snubber circuits. This may provide the reduced number of parts and a downsized switching power supply.
Other objects, structures, and advantages of the present invention will be further described in details with reference to the attached drawings as embodiments.